The present invention relates to expandable intraluminal vascular devices, generally referred to as stents. More precisely, the present invention is directed to stents that have a metallic cladding for improved expansion characteristics and radiopacity.
Stents are used to maintain patency of vessels in the body, such as a patient""s arteries. A variety of delivery systems have been devised that facilitate the placement and deployment of stents. The stent is initially manipulated while in its contracted or unexpanded state, wherein its reduced diameter more readily allows it to be introduced into the body lumen, such as a coronary artery, and maneuvered into the target site where a lesion has been dilated. Once at the target site, the stent is expanded into the vessel wall to allow fluid to flow through the stent, thus performing a scaffolding function. Stents are usually mounted on balloon catheters and advanced to a lesion site by advancing the catheter. At the site, the stent is expanded by inflating the balloon on which the stent is mounted. Deflation of the balloon and removal of the catheter leave the stent implanted in the vessel in an expanded state. It is also possible to dilate a vascular lesion and deploy a stent at the same time using the same expandable member or inflatable balloon. This variation of the procedure described above obviates the need for a separate balloon dilation catheter and stent deployment catheter.
Stents are typically formed from biocompatible metals such as stainless steel, nickel-titanium, tantalum, and the like, to provide sufficient hoop strength to perform the scaffolding function of holding the patient""s vessel open. Also, stents have minimal wall thickness in order to minimize blood flow blockage. But stents can sometimes cause complications including thrombosis, and neointimal hyperplasia by inducement of smooth muscle cell proliferation at the site of implantation of the stent. Such stents typically also do not provide for delivery of localized therapeutic pharmacological treatment of a blood vessel at the location being treated with the stent, which can be useful for overcoming such problems.
In the evolution of stents, there have been developments in the field of stents coated with a layer of polymers. The polymeric materials are typically capable of absorbing and releasing therapeutic drugs. Examples of such stents are disclosed in U.S. Pat. No. 5,443,358 to Eury; U.S. Pat. No. 5,632,840 to Campbell; U.S. Ser. No. 08/842,660, filed Apr. 15, 1997, by J. Yan; now U.S. Pat. No. 5,893,172, and U.S. Ser. No. 08/837,993, filed Apr. 15, 1997, by J. Yan.
Aside from coated stents, there have been developments in the field of multilayer grafts. An example of a multilayer graft is disclosed in U.S. Pat. No. 4,743,252 to Martin, Jr. et al. Martin et al. shows a composite graft having a porous wall structure to permit ingrowth, which graft includes a generally non-porous, polymeric membrane in the wall to prevent substantial fluid passage therethrough so as to provide an implantable porous graft that does not require preclotting prior to implantation. Grafts are known which have multiple layers for strength reinforcement. For example, U.S. Pat. No. 5,282,860 to Matsuno et al. discloses a stent tube comprising an inner tube and an outer polyethylene tube with a reinforcing braided member fitted between the inner tube and the outer tube. The inner tube is made of a fluorine-based resin.
U.S. Pat. No. 5,084,065 to Weldon et al. discloses a reinforced graft assembly made from a vascular graft component and a reinforcing sleeve component. The reinforcing sleeve component may include one or more layers. The second component of the two component system includes the reinforcing sleeve component. Like the graft component, the reinforcing component includes a porous surface and a porous subsurface. Specifically, the reinforcing sleeve component includes multiple layers formed from synthetic, biologic, or biosynthetic and generally biocompatible materials. These materials are typically biocompatible polyurethane or similar polymers.
Despite progress in the art, there is presently no stent available that has a metallic cladding for improved strength reinforcement, expansion characteristics and radiopacity. Therefore, there is a need for such a multilayer metallic clad or laminate stent.
The present invention is directed to a multilayer intracorporeal device, specifically a multilayer or laminate stent that has a metallic substrate and at least one layer of metallic cladding. The cladding is generally joined to the substrate under high pressure resulting in a structure that resists separation or delamination under normal stress. The cladding metal and the base or substrate material form a bond between them during a deep drawing, cold drawing, or co-drawing on a mandrel process. The method of combining two or more layers of different materials allows for the combination of desirable properties of those materials. Typical material properties to be considered for stent design and performance include strength, ductility, and radiopacity. For example, a substrate layer material may be chosen for its strength, a first cladding material chosen for its ductility, and a second cladding material chosen for its radiopacity.
The present invention provides a method of fabricating a stent for implantation within a body lumen, comprising the steps of providing a substrate tube having an outside surface and an inside surface; disposing a first cladding tube about the substrate tube, wherein the first cladding tube includes a metal; joining the first cladding tube to the outside surface of the substrate tube to form a laminate tube; and forming a stent pattern in the laminate tube to provide for expansion of the stent. In a preferred embodiment, the substrate tube includes a metal selected from the group consisting of stainless steel, a nickel-cobalt-chromium-molybdenum alloy, or chonichrome; and the first cladding tube includes a radiopaque metal, preferably selected from the group consisting of platinum, gold, tantalum, tungsten, platinum-10% iridium, or palladium. It may also be desirable to have a substrate tube of a psuedoelastic alloy such as NiTi. A substrate tube from such an alloy can provide mechanical characteristics which facilitate expansion of a stent within a patient""s vessels and minimize trauma to the vessels, particularly in indications such as carotid artery treatment.
Joining the first cladding tube to the outside surface of the substrate tube can include rolling and drawing the laminate tube to bond or secure the first cladding tube to the substrate tube. This process is known in the art as deep drawing, cold drawing, or co-drawing on a mandrel. Concurrent or in series with the cold drawing process, the laminate tube can be heat treated or annealed to release stress build-up from the cold working. The bond between the substrate tube and the first cladding tube can be mechanical in whole or in part.
In an alternative method, the present invention further includes disposing a second cladding tube about the first cladding tube; and joining the second cladding tube to the first cladding tube. As a result, the finished stent has two cladding layers laminated on the tubular substrate. Typically, the second cladding layer will be made of a radiopaque metal, preferably including a metal selected from the group consisting of platinum, gold, tantalum, tungsten, platinum-iridium, or palladium. A preferred platinum-iridium alloy is a platinum-10%iridium alloy.
The present invention further contemplates a device which is preferably produced by the above methods, i.e. a stent for implantation within a body lumen having a substrate tube with an exterior surface; a metallic cladding bonded under pressure about the exterior surface of the substrate tube; and a stent pattern formed in the substrate tube and the metallic cladding. In a preferred embodiment, the cladding includes a radiopaque metal, preferably selected from the group consisting of platinum, gold, tantalum, tungsten, platinum-iridium, or palladium. Furthermore, the substrate tube generally includes a metal selected from the group consisting of stainless steel, a nickel-cobalt-chromium-molybdenum alloy, or chonichrome. The substrate tube can also include a superelastic or superelastic alloy such as NiTi.
In particular, it has been found that for the substrate tube, materials such as 316L stainless steel, nickel-cobalt-chromium-molybdenum alloys such as MP35N, or cobalt-chromium-tungsten-nickel-iron alloys such as L605, (chonichrome) are preferable. For the cladding tube, it has been found that platinum, gold, tantalum, tungsten, platinum-10%iridium, or palladium are preferred. Each of the cladding material adds to the performance of the finished laminate tube which would otherwise not be possible with a pure 316L stainless steel, MP35N, or chonichrome tube alone. Another benefit of the present invention is that the metal cladded stent can have a desired amount of radiopacity. Indeed, using cladding tubes made of radiopaque alloys or metals such as platinum, gold, tantalum, or platinum-iridium increases the radiopacity of the stent to assist the cardiologist in tracking the stent during implantation.
The present invention can additionally benefit from use of a substrate or cladding tube made from nickel-titanium, which is a shape memory alloy which can exhibit superelastic properties. With a higher deformation rate due to a nickel-titanium cladding tube, the laminate stent eliminates the need for higher pressure balloons and as a result, the risk of injury to the vessel walls is reduced. The nickel-titanium. eases the expansion of the stent in normal temperatures and contraction in relatively elevated temperatures. Where a superelastic alloy such as NiTi is used as a cladding layer in combination with a non-radiopaque high strength alloy substrate such as stainless steel, MP35N or L605, it is generally preferred to include a second cladding layer or tube of a radiopaque metal such as those described above. In this way, the desired mechanical characteristics of the stent can be achieved with the appropriate combination of substrate and first cladding materials, and radiopacity is added to the stent by the second cladding layer or tube.
These and other advantages of the present invention will become apparent from the following detailed description thereof when taken in conjunction with the accompanying exemplary drawings.